Every camera has a light meter constantly measuring the scene in front of it, and metering mode determines exactly how it reads that light — which parts of the frame it weighs, and how heavily. Two photographers pointing the same camera at the same scene can get different exposure recommendations purely because their metering modes disagree about what matters in the frame.
What a Light Meter Is Actually Doing
A camera's meter doesn't know what a "correctly exposed" photo of your specific scene should look like; it works from an assumption that the scene, averaged out, is roughly middle gray, and it recommends an exposure that would render that average as middle gray. This assumption holds up reasonably well for typical scenes and breaks down for anything unusually bright or dark overall — a snow scene, a black cat, a stage lit mostly by darkness with one bright spotlight — which is exactly where understanding your metering mode starts to matter.
Matrix (or Evaluative) Metering
Matrix metering — called Evaluative on some brands — divides the frame into multiple zones, measures each one, and combines them using an algorithm that also considers focus point, color, and sometimes scene recognition. It's the default on most cameras and the right choice for the majority of everyday shooting, because it's built to handle typical scenes without you needing to think about metering at all. Where it struggles is exactly the situations described above: scenes that are legitimately very bright or very dark overall, where its assumption of "average equals middle gray" doesn't hold.
Center-Weighted Metering
Center-weighted metering measures the whole frame like Matrix does, but gives much heavier weight to the center, on the assumption that your main subject is usually there. It's a reasonable middle ground for portraits and similar subject-centered shots where you want the exposure driven mainly by the subject rather than by a busy or unevenly lit background, without going as narrow as spot metering.
Spot Metering
Spot metering ignores almost the entire frame and measures only a small area, usually tied to your focus point. This gives you precise control over exactly which part of the scene the camera treats as the reference for correct exposure, which matters a great deal in high-contrast situations. A backlit subject, a scene with the sun in frame, or a stage performer surrounded by darkness are all cases where averaging the whole frame (Matrix or Center-Weighted) would badly misjudge the exposure, while spot metering off the subject's face gives you exactly the reading you actually need.
Choosing a Mode in Practice
Leave Matrix on as your default for general shooting — it genuinely handles most scenes well and second-guessing it constantly isn't a good use of attention. Switch to Center-Weighted for portraits where you want the subject prioritized over a distracting background. Switch to Spot for high-contrast situations — backlighting, spotlit subjects, sunrise and sunset shots with the sun in frame — where you need the exposure locked to one specific part of the scene rather than an average of the whole thing. Knowing which mode you're in, and why, turns metering from a mystery the camera handles for you into a tool you're actively using.